Once anticipated as an entertaining sideshow between two feisty candidates, the vice presidential debate Thursday night has taken on higher, unexpected importance in the wake of President Obama’s listless performance last week in Denver.

Democrats are nervous, Republicans sense a surge, and Vice President Joe Biden and GOP challenger Paul Ryan suddenly have a chance to influence the campaign in a substantive way when they meet at Centre College in Danville, Ky.

For Biden, voluble and aggressive, the goal is to steady the Democratic ship amid sinking polls and rising angst. For Ryan, a hard-line budget hawk, the game plan is to maintain or build on the bump that has buoyed Mitt Romney after the first presidential debate.

“After the president’s performance last week, we know Joe Biden will be coming at us like a cannonball,” said Brendan Buck, a spokesman for Ryan.

Polls released since the Denver debate, which was viewed by 67 million people, point to a significantly tighter race. The latest national Gallup survey shows that Romney has inched ahead of Obama, 49 to 47 percent.

In addition, the president’s once-growing lead has narrowed in several swing states, including the critical battleground of Ohio, with Obama leading by just 4 points, according to a CNN poll released Tuesday.

Suffolk University pollster David Paleologos said Tuesday he has stopped polling in Virginia, Florida, and North Carolina because he said he believes Obama will lose those states.

The task for Biden, 69, is greater than simply avoiding the kind of verbal gaffes that he has committed in the past. The vice president, an aging veteran who served 36 years in the Senate, has been summoned from the bullpen to slow or stop the GOP’s rally.

“It’s a continuation of the presidential debate,” said Todd Domke, a Republican consultant who is a political analyst for WBUR. “The Democrats hope that a Biden victory will establish the first presidential debate as an aberration, and the Republicans hope a Ryan victory will prove that the GOP has the momentum and the arguments for change.”

For a vice presidential debate, Domke said, such stakes are unprecedented.

Obama campaign officials predicted that Biden, who spent time this week preparing at his Delaware home, would be focused and effective.

“There’s no more passionate advocate for the administration’s approach to the last four years to the challenges the middle class is facing. And we expect he’ll make the case for sending the president and himself back for another four years,” Obama campaign spokeswoman Jen Psaki told reporters on Air Force One.

Many observers expect Biden to question Ryan, 42, about hot-button topics that Obama surprisingly avoided in Denver, including Romney’s videotaped dismissal of the “47 percent” of Americans who receive government assistance.

If he does not raise these issues, Domke said, voters will ask, “ ‘What is going on? They spend hundreds of millions of dollars advertising this stuff, and then they don’t mention it?’ It’d be political malpractice.”

The vice president also is expected to attack Ryan’s controversial budget proposal, which would add a voucher option to Medicare and dramatically pare spending on social services.

For his part, Ryan is expected to continue hammering away with the Republican campaign theme that Obama has failed to fashion an economic recovery, the deficit has soared to dangerous and unsustainable levels, and his foreign policy, particularly toward the Middle East, has been meandering and weak.

Ryan is preparing for a fight.

In an interview on a Michigan radio show this week, Ryan said he expected that Obama and Biden “are just going to call us liars for a month . . . to descend down into a mud pit and, hopefully, with enough mudslinging back and forth and distortion, people will get demoralized and then they can win by default.”

The 90-minute debate, which will be moderated by foreign affairs correspondent Martha Raddatz of ABC News, will begin at 9 p.m. and include questions on domestic and foreign issues.

Marc Landy, a Boston College political science professor, said the debate will occur amid a “vulture phenomenon,” in which the news media and TV comics have seized on Obama’s debate performance with a kind of car-wreck fascination.

For Democrats, Landy said, “it’s really important for Biden to change the story line, to get the press talking about something else so Obama can regain his footing.”

Ryan, however, should be a formidable opponent, Landy said.

“He is extremely knowledgable and makes a good presentation,” Landy said. “He’s not going to fall over for Biden. My sense is that Ryan will perform quite well in the debate.”

Perhaps because of his unpredictability, Biden attracts an audience. His 2008 debate with Sarah Palin is tied with the Perot-Clinton-Bush debate of 1992 as the second-most-watched debate — presidential or vice presidential — in history. (First place goes to Jimmy Carter and Ronald Reagan’s in 1980.) This year, Biden recorded the highest viewer ratings of any speaker at the Democratic and Republican national conventions.

In all, Biden has participated in 18 presidential or vice presidential debates, including 14 in 2008.

Despite his populist persona, more voters view Biden unfavorably than favorably, according to a poll released this week by the Pew Research Center for the People & the Press. The survey found that 39 percent had a favorable impression of Biden, while 51 percent viewed him unfavorably.

The Wisconsin congressman “is unusual in being able to impress by being a policy wonk and also sounding very down to earth,” Domke said. “The test for Ryan will be, ‘Yeah, he didn’t seem defensive, he had answers, he wasn’t thrown by this guy.’ ”

Ryan spent five days in debate rehearsals over the last two weeks in addition to reading every day to prepare, a campaign official said.

Ted Olson, who served as solicitor general for President George W. Bush, played Biden in those sessions.

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